There are a variety of arrangements that incorporate a TSV die in a vertical stack to take advantage of the two (2)-sided connectivity provided by TSV die. For example, package-on-package (PoP) is one type of assembly flow that incorporates a TSV die in a vertical stack. Other examples include a PoP precursor, as well as some non-PoP packages, such as a package substrate on bottom, logic with TSV bonded to that, and a memory stack bonded to that, which may ship with no additional ball grid array (BGA)-containing package added after that.
For example, PoP is an IC packaging technique that allows vertical stacking of IC packages, such as a discrete logic BGA package and a memory BGA package. Two or more packages are installed on top of one another, i.e. vertically stacked, with a standard interface to route signals between them. This allows higher density, for example for mobile telephone/PDA market applications.
TSV-containing IC die such as microprocessor TSV die in a PoP flow are generally not yet in production. However, the assembly flow for a conventional PoP sequence for a wafer having a plurality of TSV die (a “TSV wafer”) can be expected to be as follows:
1. Wafer probe (“multiprobe”) the respective die on a thick TSV wafer (e.g., 600 to 800 μm thick) having embedded TSVs to identify good die on the basis of electrical tests. Electronic Inking where the die are tracked on stored wafer maps may be used for subsequent identification of “bad” die.2. Attach the TSV wafer to a carrier wafer (typically silicon or glass).3. Expose the embedded TSVs by thinning the bottom side of the TSV wafer (e.g. to about 30 to 100 μm) that may comprise protruding integral TSV tips that protrude <15 μm.4. Add metal finish or contact pads coupled to exposed TSV tips.5. Detach the carrier wafer.6. Singulate the TSV wafer to provide a plurality of singulated “good” TSV die.7. Die attach the good TSV die identified at wafer probe active circuit side down to a multi-layer (ML) package substrate that includes BGA pads on its bottom side that is typically attached to a substrate carrier (typically a silicon or glass carrier) to form a PoP precursor. The carrier provides rigidity. The active circuit side (i.e. frontside) of the TSV die is not electrically accessible via the package substrate due to the presence of the carrier.8. Attach one or more die, such as a logic or memory die, on top of the TSV die to make contact with the metalized TSV tips or contact pads coupled to the TSV tips.
Steps 3, 4, and 7 in the above-described flow can result in electrical problems including TSV formation problems (e.g., missing TSVs), TSV contact problems (e.g., high resistance contacts to pads on the ML package substrate) or shorts (e.g., TSV shorts to ground) that can only be detected after connecting the top die to the exposed TSVs or to the contact pads coupled to the TSV tips because as noted above the carrier while present blocks electrical access to one side of the TSV die. The ML package substrate can also be the source of certain problems. Probing after die attach of the TSV die can be omitted. However, this will result in assembling some fraction of bad TSV die-substrate precursors during step 8 described above for attaching subsequent logic or memory die (e.g., where costly pre-packaged memory stacks may be added), that due to electrical problems such as those described above can result in failures at post-assembly testing.
There are no known practical solutions to test partially assembled TSV-die-on-substrate die stacks to evaluate joint integrity between the TSV die and the package substrate associated with the die attach process. Even if it were possible to contact probe the TSV die contacts to the ML package substrate using the protruding TSV tips from the bottomside of the TSV die for the topside contacts and some other contact pathway for contacting the bottomside contacts, standard contact probe techniques tend to damage the protruding (e.g. <15 μm) TSV tips, which can lead to an unreliable joint at the site of the damaged TSV tip following assembly of the top die to the TSV die. Such damage from contact probing of TSV tips can lower bond yield and/or reduce TSV joint reliability. What is needed is a non-contact method for determining TSV continuity through the TSV die and joint integrity between pillars or microbumps coupled to the TSVs on the TSV die to the underlying package substrate for partially assembled precursors.